Auxin is an essential regulator for almost every aspect of plant growth and development. The general goal of this proposal is to elucidate the molecular mechanisms by which auxin regulates plant organogenesis and other developmental processes. During the last grant period, we demonstrated that the YUCCA (YUC) flavin monooxygenases play an essential role in auxin biosynthesis and various developmental processes including embryogenesis, seedling growth, vascular patterning, and floral development. Previous genetic studies on auxin signaling were centered on analysis of mutants resistant to exogenous auxin. Our understanding of auxin biosynthesis allowed us to elucidate the molecular mechanisms of auxin action in plant development from a completely different perspective. We identified npy1 (naked pins in yuc mutants) from a genetic screen for enhancers of the yuc1 yuc4 double mutants that are partially auxin deficient. The npy1 yuc1 yuc4 triple mutants developed pin-like inflorescences and failed to form any flowers, a hallmark phenotype caused by defects in auxin pathways. NPY1 belongs to a large family and is homologous to non-phototropic hypocotyl 3 (NPH3), a BTB protein regulating phototropism along with the AGC kinase PHOT1. NPY1 works with PID, a PHOT1 homolog, to regulate auxin-mediated organogenesis using a mechanism analogous to that used by NPH3/PHOT1 in phototropism. The findings put yuc, npy1, and known auxin mutants pin1, pid, and mp in a genetic framework for further understanding of the mechanisms governing auxin-regulated plant development. The findings also reveal a novel mechanism for AGC kinase-mediated signal transduction in plants. The primary aims of the proposed studies are (1) Analysis of the mechanisms of YUC genes in auxin biosynthesis and plant development; (2) Elucidation of the biochemical mechanisms of NPY1 in auxin-regulated organogenesis; (3) Determination of the unique and overlapping functions of the NPY1 family genes in auxin-regulated plant development; (4) Genetic dissection of the mechanisms by which auxin regulates plant development. The proposed experiments will provide significant new insight into the molecular mechanisms of auxin action in plant development, particularly in organogenesis. A clear understanding of the mechanisms by which auxin regulates complex developmental processes is of fundamental importance to plant biology and will have a significant agricultural impact. The proposed study will also extend our understanding of signaling mechanisms controlling complex developmental processes in other eukaryotes, particularly the mechanisms of AGC kinases, which have been implicated in diverse developmental programs in other organisms including humans. PUBLIC HEALTH RELEVANCE: The proposed research is aimed to elucidate the molecular mechanisms by which auxin controls the formation of plant organs. The proposed research will augment our understanding of complex signal transduction mechanisms governing organogenesis and other developmental processes in eukaryotes including humans.